The question has arisen concerning the use of balsa vs. foam core in decks for a mid-size (80 ft.) cruiser-style boat. Is there a formula or some place I can locate info on the deflection and tensile strength between the two? Also, how about the weight differences of these materials?

The manufacturers of the core products (Baltek for balsa) publish engineering data on their products. But it's probably not the core properties that are going to drive strength/deflection on the decks. It's the glass reinforcements that are laminated on the outsides of the core - that's where the load is carried. Similar to an I-beam - the web is the core, and the flanges are the laminate. They work together as a system.

Typically foam can be lighter than balsa, but there's lots of different foam out there.

Anybody tried this product from spheretex? "sphere.core SBC is a newly developed mat consisting of volumised short glass fibers"
We have recently replaced endgrain balsa stiffening in our 10m hulls (fast planing fishing boats) with 10mm in the bottom planes and 6mm in the sidewalls.
It weighs about the same but gives a more homogeneous panel which can take hard shocks without delamination.

But it's probably not the core properties that are going to drive strength/deflection on the decks. It's the glass reinforcements that are laminated on the outsides of the core - that's where the load is carried. Similar to an I-beam - the web is the core, and the flanges are the laminate. They work together as a system.

The strength of the core IS important. It is the core which keeps the laminates apart. Balsa (end grain) is much stronger in compression and sheer than any of the foams. To achieve similar rigidity to a balsa core in a panel using a foam core will require a thicker core, and /or thicker glass laminates. The results are that a balsa cored panel can be lighter for a given stiffness than a foam cored one.

Compressive strength? That is precisely what is wrong with balsa in highly stressed areas it imparts the impact to the inside laminate! Hard to beat a “ductile foam” like airex or core cell ..Stiffness! Well that is great but taken to the point of yield a different story you have what ; failure better to give a bit and keep the panel intact!

Compressive strength? That is precisely what is wrong with balsa in highly stressed areas it imparts the impact to the inside laminate! Hard to beat a “ductile foam” like airex or core cell ..Stiffness! Well that is great but taken to the point of yield a different story you have what ; failure better to give a bit and keep the panel intact!

There is a difference between stress and impact. In a stress loaded area it is important to keep the laminates apart, for rigidity. In an impact situation it really depends on how large the impact is : balsa will resist damage from small impacts better - it will support the outer laminate better than foam will. In large impacts it's true foam will transmit less of the force to the inner laminate, but the outer laminate and core will suffer greater damage.

Stiffness! Well that is great but taken to the point of yield a different story you have what ; failure better to give a bit and keep the panel intact!

The reason we use cores of any type is to increase stiffness, without adding weight. The desired stiffness of a panel could also be acheived by a solid layup, and often is, but at far greater weight than sandwich construction. So what we are looking for from a core material is the best possible panel stiffnes per weight. Panel stiffness comes from the cores ability to resist compression and sheer, and balsa is superior to foam in these properties. It is denser, but to get the same stiffness from a foam core requires a thicker laminate, or a thicker core, or both, and results in a heavier panel. ATL in Queensland make Epoxy/balsa/epoxy panels, as well as epoxy/foam/epoxy panels. They have comparitive data for them on their website. www.atlcomposites.com.au